1. Field of the Invention
The present invention relates to an electronically controlled fuel injection system for an internal combustion engine mounted in an automotive vehicle, particularly applicable to an engine of a construction wherein an extremely lean air-fuel mixture is burned.
2. Description of the Prior Art
A conventional electronically controlled fuel injection system applied to a four-cylinder engine comprises: (a) a plurality of fuel injection valves, each provided at an inlet port of an intake manifold of an engine body; (b) a throttle valve opening sensor attached to an axis of a throttle valve located within a throttle chamber interlinked with the intake manifold for detecting the fully-open position of the throttle valve; (c) an acceleration wire interlinking an accelerator pedal with the throttle valve so that the angle of inclination of the accelerator pedal corresponds to the angle of opening of the throttle valve; (d) an intake air quantity sensor located between the throttle valve and an air cleaner; and (e) a water temperature sensor for detecting the cooling water temperature of the engine. The conventional electronically controlled fuel injection system further comprises a control unit which measures a number of engine revolutions by counting the number of ignition pulses, each generated whenever a minus terminal of an ignition coil produces an ignition pulse, calculates the amount of fuel to be injected into the engine according to engine operating conditions from data on intake air quantity and engine cooling water temperature obtained by the intake air quantity sensor and the engine cooling water temperature sensor in order to derive the pulse width (valve opening time) of a drive pulse signal to be applied to each fuel injection valve. In addition, when the throttle valve is fully opened, the fuel injection amount is incremented by means of the control unit in response to an ON signal from the throttle valve opening sensor.
However, since in conventional electronically controlled fuel injection systems, the angle of inclination of the accelerator pedal corresponds to the throttle valve opening angle and the fuel injection amount is incremented at the full-load position of the throttle valve at which the angle of inclination of the accelerator is maximized, i.e., when the throttle valve is fully opened, in the case of a lean air-fuel mixture buring engine which runs on a lean air-fuel mixture ratio greater than 18 during partial-load driving, the air-fuel mixture would rapidly be changed to a rich mixture when the engine is switched from a partial-load condition to a full-load condition so that the vehicle traveling speed may change abruptly and vehicle handling be thereby worsened.